How old is your cover? I've had this happen twice now with our 13 year old aquamatic cover. For us what happens is either the flange corrodes or the last time the drum the cover rolls up on has corroded until it is no longer connected to the part of the flange attached to the axle. So the axle is turning part of the flange, or even the whole flange but the drum isn't turning because it isn't connected anymore. If your flange is turning on the motor side, check the non-motor side to see if it is turning too, if not that is your problem. The pool company was able to make a quick repair this spring and get us going for the season, but next spring we plan to replace the cover (it's leaking quite a bit this year), and while they are in there they will replace drum and anything else that has bad corrosion. Not looking forward to that bill.
